Compound lumber slabs, made from lengths of lumber edge-glued together, are used in the wood products industry both as an end product and as an intermediate step in the production of wood paneling. Wood paneling is produced from such slabs by resawing the slab along planes parallel to its face to yield two or more thinner panels. Although the basic concepts of compound lumber and resawn panels are over a half-century old, recent advances in wood-bonding technology have made possible compound wood products that rival one-piece wood products in structural integrity.
The basic concept of compound lumber is disclosed in U.S. Pat. No. 1,924,240 to Harwell. A plurality of short lengths of lumber are glued edge-to-edge and end-to-end to form a single slab. The slab can then be cut along the plane of the slab to form two panels. Harwell's edge joints are formed by a finger on the edge of one lumber piece that mates with a groove in the edge of the adjoining lumber piece to form the slab. When the slab is resawn into two panels, the edge joint becomes a simple lap joint, which is notoriously weak.
Other types of joints are also used in the wood products industry. U.S. Pat. No. 161,746 to Brock, for example, discloses a method of joining two layers of wood using interengaging ribs and grooves on the two adjoining surfaces. The ribs of Brock are truncated triangles having one side normal to the plane of the wood and one side inclined. The width of the ribs is slightly larger than the width of the grooves, so that external pressure is required to force the two boards together. If the forcing is not complete, there will be a visible gap between the two joined pieces where their surfaces meet. The high strength of the joint is primarily attributed to the intimate frictional contact between the layers of wood rather than to the glue.
Other methods for forming wood panels from component wood pieces are also known in the lumber art. U.S. Pat. No. 1,720,841 to Jones discloses a method for producing a board of any width by joining together a plurality of thin lumber pieces by using lap joints. The rabbeted thin stock lumber is produced by notching the edges of thick stock lumber and cutting the thick stock lumber along planes that bisect the notches before lap-bonding the pieces together.
The zigzag or "sawtooth" joint and common butt joint are presently the techniques most used for edge joining a plurality of lumber pieces into a compound slab for resawing into door panels. A sawtooth joint is typically stronger than a butt joint. For a sawtooth joint, a complementary zigzag pattern is cut into the edges of adjoining lumber pieces, which are then mated and glued together. Slight imperfections in the alignment of the zigzags, or deviations in the path of a saw blade from a straight line through the slab, however, can produce pronounced nonparallel glue lines on the faces of the resawn panels cut from such slabs, detracting from the appearance of such panels. In addition, zigzag joints must be pressed together and bonded under high pressure because of the interference fit commonly provided between the fingers and grooves. If pressure is insufficient, gaps may appear along the joint line on the surfaces of resulting panels.
Although most of the foregoing methods for forming compound lumber panels can be used to produce resawn panels of any desired width, the structural integrity of the resulting panels is sometimes inadequate. Wood panels used in exterior doors, for example, must afford some measure of security. The weakness of prior butt and lap joints makes them unsuitable in such applications.
Compound wood panels used in finished wood product, such as doors, must also have a pleasing appearance. As noted, the zigzag and other prior finger-type joints can produce resawn panels with noticeably nonparallel glue lines or gaps in such lines. Such panels are unsuitable for applications, such as for door panels, where appearance is important.
Accordingly, a need remains for a compound wood slab that, when resawn, yields panels with a high degree of structural integrity as well as a pleasing appearance.